Implantable medical devices are available to provide therapies for restoring normal cardiac rhythms by delivering electrical shock therapy for cardioverting or defibrillating the heart, in addition to providing cardiac pacing. Such a device, commonly known as an implantable cardioverter defibrillator (“ICD”) senses a patient's heart rhythm and may classify the rhythm according to a number of programmable rate zones in order to detect episodes of tachycardia or fibrillation. Single chamber devices are available for treating either atrial arrhythmias or ventricular arrhythmias, and dual chamber devices are available for treating both atrial and ventricular arrhythmias. Rate zone classifications may include slow tachycardia, fast tachycardia, and fibrillation.
Upon detecting an abnormal rhythm, the ICD may select and deliver a therapy based upon detected rate and/or other programmable criteria, for example. Cardiac pacing may be delivered in response to the absence of sensed intrinsic depolarizations within a specified time window, referred to as P-waves in the atrium and R-waves in the ventricle. In response to tachycardia detection, a number of tiered therapies may be delivered beginning with anti-tachycardia pacing therapies and possibly escalating to more aggressive therapies until the tachycardia is terminated. Termination of a tachycardia is commonly referred to as “cardioversion.” Ventricular fibrillation (VF) is a serious life-threatening condition and is normally treated by delivering high-energy shock therapy. Termination of VF in this manner is normally referred to as “defibrillation.”
In many currently available ICDs, a physician or operator has the ability to program particular anti-arrhythmia therapies into the device ahead of time, and a menu of therapy options is typically provided. For example, on initial detection of an atrial or ventricular tachycardia, an anti-tachycardia pacing therapy may be selected and delivered to the chamber or chambers in which the tachycardia is diagnosed. After the initial therapy is delivered, a subsequent redetection of tachycardia may lead to a more aggressive anti-tachycardia pacing therapy, for example. If repeated attempts at anti-tachycardia pacing therapies fail, a cardioversion or defibrillation shock may next be selected. For an overview of tachycardia detection and treatment therapies, reference is made to U.S. Pat. No. 5,545,186 issued to Olson et al.
Detection of tachycardia or fibrillation may also trigger the storage of sensed intracardiac electrograms (EGMs) for a period of time such that the EGM signals leading up to and during a detected arrhythmia episode may be available for downloading from the ICD and displaying on an external programmer or other device for analysis by a physician. Such analysis of stored EGM signals may aid the physician in monitoring the status of the patient and the patient's response to delivered therapies. Occasionally, an ICD may inappropriately detect a tachycardia or fibrillation episode that does not exist physiologically, and may deliver an anti-arrhythmia therapy when one is not needed or desired. Inappropriate arrhythmia detections may, in some cases, cause a patient to experience painful, repeated cardioversion or defibrillation shocks within a relatively short period of time. Certain therapies delivered inappropriately, for example during normal sinus rhythm, can potentially induce an arrhythmia in some patients. For these reasons, the delivery of a therapy in response to inappropriate arrhythmia detection is highly undesirable.
Inappropriate arrhythmia detection may be caused by “oversensing.” Oversensing may be defined as the sensing or detection of cardiac events other than what should be expected for a given physiological condition. Oversensing of both cardiac and non-cardiac events can result in inappropriate arrhythmia detection by the ICD if the detected rate due to oversensing falls into an arrhythmia detection rate zone. Cardiac oversensing typically refers to oversensing of cardiac events such as far-field R-waves, T-waves, or R-waves that are sensed twice and are therefore “double-counted.” Examples of cardiac oversensing are illustrated in FIG. 1. A conventional electrocardiogram (“ECG”) signal is illustrated showing a normal cardiac cycle indicated by a P-wave, R-wave, and T-wave. Beneath the ECG is a representation of a ventricular intracardiac electrogram signal (VEGM) in which a ventricular signal spike coincides in time with the R-wave on the ECG. During normal sensing, a patient in normal sinus rhythm will exhibit one atrial sensed event (AS) and one ventricular sensed event (VS) during each cardiac cycle, corresponding to the atrial P-wave and the ventricular R-wave, respectively, as indicated by the marker channel labeled “Normal Sensing” (shown beneath the VEGM). The VEGM is typically obtained by recording inputs from a pair of relatively closely-spaced electrodes located in proximity to a ventricle of the heart. Signals from such closely-spaced electrodes are sometimes referred to as “near-field” electrogram (NF EGM) signals. The spacing of electrodes for obtaining NF EGM signals may be determined, for example, by the tip-to-ring electrode spacing of commonly used bipolar electrodes, as are known in the art.
Far-field R-wave oversensing (sensing of ventricular depolarizations by the atrial lead) is illustrated in FIG. 1. In the marker channel labeled “Far Field R-wave Oversensing,” two atrial sensed events (AS) are indicated during each cardiac cycle. One atrial sensed event (AS) per cardiac cycle corresponds to the appropriate sensing of a P-wave, while a second atrial sensed event (AS) per cardiac cycle corresponds to the far-field sensing of the R-wave. Far-field R-waves are sometimes sensed on the atrial EGM (not shown) because the amplitude of an R-wave, as sensed at the atrial sensing electrodes, may reach or exceed the atrial sensitivity threshold. Therefore, an atrial sensitivity setting that appropriately senses P-waves may sometimes also result in inappropriate sensing of far-field R-waves from the ventricles (i.e., oversensing).
T-wave oversensing is also illustrated in FIG. 1. In the marker channel labeled “T-wave Oversensing,” two ventricular sensed events (VS) are shown occurring during each cardiac cycle, one coinciding in time with the R-wave and one coinciding in time with the T-wave. T-wave oversensing may occur when the ventricular sensitivity setting is too sensitive, for example, resulting in sensing of both R-waves and T-waves. T-wave oversensing may also occur when the R-wave amplitude has decreased sufficiently to cause the “auto-adjusting threshold,” which varies the ventricular sensitivity setting as a function of the sensed R-wave amplitude, to decrease below the T-wave amplitude. R-wave oversensing, also referred to as “R-wave double-counting,” is also illustrated in FIG. 1 in which two ventricular sense events (VS) correspond to a single R-wave. This “double-counting” of R-waves can occur, for example, when an R-wave complex is widened due to conditions such as bundle branch block or wide complex ventricular tachycardia. For each of the above-described types of cardiac oversensing, generally one extra atrial or ventricular sensed event may be detected per cardiac cycle, as seen in the illustrations of FIG. 1.
Non-cardiac oversensing refers to undesired sensing by an ICD of electrical signals that are not cardiac in origin. Such non-cardiac signals are sometimes referred to as “noise.” Noise may occur in the form of myopotentials (electrical signals generated by surrounding muscle tissue) or as the result of electromagnetic interference (“EMI”) from sources external to the patient. Noise may also occur when the insulation of a lead fails, when a lead conductor becomes fractured, or when a lead is poorly connected to the ICD.
Examples of non-cardiac oversensing are illustrated in FIGS. 2A through 2C. In FIG. 2A, a ventricular EGM signal is shown with a corresponding illustration of electromagnetic interference (EMI) oversensing. EMI appears as relatively continuous high frequency noise on the VEGM and can be repeatedly detected as a ventricular sensed event (VS) by the ICD. In FIG. 2B, a ventricular EGM is shown with a corresponding illustration of myopotential oversensing. Myopotentials may appear as noise on the VEGM at lower frequencies than EMI, resulting in somewhat less frequent but repeated ventricular sensed events (VS). In FIG. 2C, a ventricular EGM is shown corresponding to noise associated with a lead fracture or a poor lead connection. This type of noise can result in saturation of the sense amplifiers and intermittent bursts of noise. Oversensing due to a lead fracture or poor lead connection, therefore, may produce intermittent clusters of ventricular sensed events (VS), as shown in FIG. 2C. As seen in FIGS. 2A through 2C, non-cardiac oversensing is generally associated with multiple oversensed events in an affected cardiac cycle, and may be intermittent or continuous, of high or low amplitude, and of relatively low or high frequency.
Since the various types of oversensing may occur relatively infrequently and are not routinely encountered in all patients, the task of recognizing and trouble-shooting oversensing conditions can be a challenging one to the physician. Oversensing may not be recognized until inappropriate arrhythmia detections are made and/or unneeded therapies delivered. While stored EGM data can be useful in identifying and trouble-shooting inappropriate arrhythmia detections due to oversensing, valid arrhythmia detections may occur the majority of the time with only an occasional inappropriate detection occurring; this may make the task of identifying the cause of inappropriate detections from analysis of stored EGM episode data a difficult and time-consuming task. Once an inappropriate detection is identified, the numerous types of oversensing that may have caused the inappropriate detection make diagnosing the problem complex. With a growing number of ICD patients in broad geographical distributions, clinicians need to be able to quickly and confidently diagnose and correct such problems. What is needed, therefore, is an automated method for recognizing oversensing and specifically identifying the type of oversensing present so that a physician may make prompt corrective actions with confidence.